The present invention relates to semiconductor chips mounted on a substrate. It finds particular application in conjunction with compensating for different spectral characteristics between the chips and will be described with particular reference thereto.
Image sensor arrays for scanning original documents, such as Charge Coupled Devices (CCD's), typically have a row or linear array of photosites together with suitable supporting circuitry integrated onto a silicon substrate. Usually, an array of this type is used to scan, line by line, across the width of a document with the document being moved or stepped lengthwise in synchronism therewith.
A linear array of small photosensors are commonly provided in a full-page-width image scanner. The photosensors extend the full width of an original document (e.g., eleven (11) inches). These photosensors are spaced as finely as 600 to the inch on each chip. In a document scanner, optical means, e.g., a lens, is used to form an original image of the light reflected from the original document. When the original document moves past the linear array, each of the photosensors converts light from the original image into electrical signals. The motion of the original document perpendicular to the linear array causes a sequence of signals to be output from each photosensor. These output signals are converted into digital data.
The full-width array bar is typically assembled from a number of semiconductor sensor chips butted against one another to form a colinear array. The sensor chips are fabricated in groups, with many chips being formed on a single silicon wafer. The wafers, in turn, are processed in batches. It is not uncommon for the sensor chips in a full-width array to be selected from different wafers and different batches. Although the variations from batch-to-batch and wafer-to-wafer are ideally controlled, chip-to-chip differences do exist because of inherent tolerances in microelectronics fabrication. Therefore, when the chips are butted next to each other on the bar, the chips can produce color differences and/or banding in the scanned output image.
The currently-preferred design for creating a long linear array of photosensors includes using a set of relatively small semiconductor chips. Each semiconductor chip defines thereon a linear array of photosensors along with ancillary circuit devices. These chips are typically approximately 0.62 inches in length. Therefore, in order to create a practical full-page width array, as many as twenty (20) or more of these chips are abutted end-to-end to form a single linear array of photosensors.
The abutted chips are typically mounted on a support platform. This support platform also includes circuitry, such as on a printed wire board, which accesses the circuit devices on the individual chips for a practical system. The interconnections between the relatively large-scale conductors on the printed wire board and the relatively small contact pads on the semiconductor chips are preferably created by wire bonds. Typically, these wire bonds are ultrasonically welded to both the printed wire board conductors and to contact pads on the chips.
One advantage of a system using a single, color sensor chip for capturing a full scanline of data is that red, green and blue ("RGB") filters, typically placed on the photosite areas within the wafer, are well controlled. However, as referenced above, one drawback to such an arrangement is that the semiconductor chips forming a full-width array typically do not all have similar filter characteristics. Such a phenomenon is caused by several factors. For example, semiconductor chips are commonly produced in batches. It is not unusual for chips produced in one batch to have unique colorant and spectral characteristic profiles. These different profiles may be caused by different filter thicknesses occurring from one batch to the next because of various photolithographic techniques which are used to produce the chips. Differences in the thickness and spectral characteristic profiles both play an important role in the final colormetric output from one semiconductor chip to another. More specifically, different spectral characteristics between various chips on the sensor bar may cause color banding to occur in the final picture.
One currently used method for determining spectral characteristics between various chips on the sensor bar is to measure the filter thicknesses of the wafers on each chip. The chips are then pre-sorted according to the thicknesses of the wafers. Although this method results in a linear array including chips having wafers of similar thicknesses, the process of measuring and sorting the filters is very expensive and time consuming.
The present invention provides a new and improved apparatus and method which overcomes the above-referenced problems and others.